Dr. Christopher M. White
Professor
Department of Mechanical Engineering
University of New Hampshire
Friday, February 20, 2026, 3:10pm
Chase 105
Abstract
The dispersion of scalars from the free end of wall-mounted obstacles is a fundamental problem in fluid mechanics, with direct implications for industrial leak detection and urban pollutant transport. This study presents a comprehensive experimental investigation of scalar plumes immersed in a large-Reynolds-number turbulent boundary layer. Unlike idealized point sources, plumes originating from the wake of cylindrical obstacles are governed by complex interactions between the source geometry, the shear layer, and the wall.
We systematically vary the aspect ratio (AR), diameter ratio (n), and velocity ratio (r) to characterize the evolution of the scalar field. A central contribution of this work is the classification of transition behavior between three distinct dispersion regimes: Elevated Plumes (EP), Ground-Level Plumes (GLP), and Ground-Level Sources (GLS). We demonstrate that these transitions are physically governed by the non-dimensional parameter which relates the inferred effective source height to the vertical plume half-width. This parameter serves as a quantitative metric for the progressive "grounding" of the plume.
High-resolution concentration measurements reveal that as the plume transitions through these regimes, both vertical and lateral dispersion characteristics exhibit systematic departures from classical dispersion-coefficient scaling. To assess the predictive limits of current frameworks, the experimentally derived dispersion coefficients and effective source heights are used as inputs for the Gaussian Dispersion Model (GDM) and a Wall Similarity Model (WSM). Our results show that while the GDM captures general far-field trends, it struggles with the non-linearities of the transitional regimes. In contrast, the WSM provides a superior representation under GLS conditions where wall-bounded turbulence dominates transport.
Finally, we integrate mean and turbulent velocity measurements to provide a physical framework for these observations. We show how coherent vortical structures shed from the cylinder's free end modulate the scalar transport, shaping the plume's trajectory and growth. This dataset establishes a critical benchmark for validating CFD simulations and informing the next generation of predictive dispersion models in complex turbulent flows.
Bio
Christopher White received his B.S. in mechanical engineering from Stony Brook University, and his Ph.D. from Yale. He was then a postdoctoral scholar at Stanford before becoming a staff scientist at Sandia National Lab. He joined UNH some 20 years ago and has been serving as the department chair for mechanical engineering since 2020. His research interests include experimental investigation on high-speed turbulent flow dynamics.
